1. Field of the Invention
The invention pertains to semiconductor devices which include superconducting interconnects.
2. Art Background
A superconducting material is one which exhibits zero resistance to the flow of DC electrical current. Such a material is characterized by a temperature, called the critical temperature (T.sub.c), above which the material ceases to be superconducting.
Prior to 1986, the known superconducting materials included elemental metals, such as Hg, intermetallic compounds, such as Nb.sub.3 Ge, as well as a few (non-cuprate) metal oxides, such as BaPb.sub.1-x Bi.sub.x O.sub.3, where 0.05.ltoreq..times..ltoreq.0.3. Significantly, all these materials have T.sub.c s which are less than or equal to about 23.3 Kelvins (K.). As a consequence, it is necessary to use relatively expensive liquid helium to cool these materials below their respective T.sub.c s to achieve superconductivity, which makes the use of these superconductors relatively expensive.
In 1986, J. G. Bednorz and K. A. Muller published their now seminal discovery that compositions in the La-Ba-Cu-O system are superconducting and have T.sub.c s as high as about 30 K. (See J. G. Bednorz and K. A. Muller, Zeitschr. f. Physik B-Condensed Matter, Vol. 64, 189 (1986).) This discovery stimulated scientists all over the world to look for compositions having even higher T.sub.c s resulting, among other things, in the discovery by C. W. Chu and colleagues that mixed phase compositions in the Y-Ba-Cu-O system have T.sub.c s as high as about 93 K. (See M. K. Wu et al, Physical Review Letters, Vol. 58,908 (1987); and P. H. Hor et al, Physical Review Letters, Vol. 58,991 (1987).) This latter discovery generated considerable excitement because the latter compositions are readily cooled below their T.sub.c s with relatively inexpensive liquid nitrogen (which has a boiling point of 77 K.), making their use as superconductors relatively inexpensive and thus potentially commercially attractive. This discovery, in turn, was followed by the idenification by R. J. Cava and colleagues of YBa.sub.2 Cu.sub.3 O.sub.7 as being the particular phase in the Y-Ba-Cu-O system responsible for the high temperature superconductivity. (See R. J. Cava et al, Physical Review Letters, Vol. 58, 1676 (1987).)
To date, the research efforts of Bednorz and Muller, and those they inspired, has resulted in the identification of two classes of copper oxide superconductors. The first class has nominal composition La.sub.2-x M.sub.x Cu O.sub.4-.epsilon., where M denotes one or more divalent metals, such as Ba, Sr or Ca, 0.ltoreq..times..ltoreq.0.3 and 0.ltoreq..epsilon..ltoreq.0.1. (See R. J. Cava et al, Physical Review Letters, Vol. 58,408 (1987); and K. Kishio et al, Chemistry Letters, 429 (1987).) The members of this first class have been found to have T.sub.c s ranging from about 30 K. to about 40 K.
The second class of copper oxide superconductors has nominal composition Ba.sub.2-y (M.sub.1-x.sup.(1) M.sub.x.sup.(2)).sub.1+y Cu.sub.3 O.sub.9-.delta., where 0.ltoreq..times..ltoreq.1, 0.ltoreq.y.ltoreq.1, 1&lt;.delta.&lt;3 and each of M.sup.(1) and M.sup.(2) denotes Y, Eu, Nd, Sm, Gd, Dy, Ho, Er, Tm, Yb, Lu, La, Sc, Sr or combinations of these elements. (See D. W. Murphy et al, Physical Review Letters, Vol. 58, 1888 (1987); P. H. Hor et al, Physical Review Letters, Vol. 58, 1891 (1987); and presentation given by J. M. Tarascon et al at Materials Research Society Meeting, Anaheim, California, April 1987). Significantly, many of the members of this second class have T.sub.c s greater than about 77 K. (the boiling point of liquid nitrogen.).
Just recently, there have been reports that partial or complete substitution of fluorine for copper in the second class of copper oxide superconductors also yields superconductors with T.sub.c s greater than about 77 K. (See S. R. Ovshinsky et al, Physical Review Letters, Vol. 58, 2579 (1987).)
The discovery of superconductors having relatively high T.sub.c s, e.g., T.sub.c s greater than about 77 K., has led to a wide variety of proposed applications. One such proposal is to use (liquid nitrogen-cooled) superconductors as interconnects (electrical conducotrs extending, and used for transmitting signals, between device components) on and between semiconductor chips. (See, e.g., the newspaper article by Andrew Pollack entitiled, "Standford Reports Advance In Race for Supercomputer," The New York Times, page 7, Mar. 14, 1987; and the newspaper article by James Gleick entitled, "New Superconductors Offer Chance to Do the Impossible," The New York Times, page 1, Apr. 9, 1987.) One of the advantages underlying this proposal is the fact that such use would reduce, or even entirely eliminate, the RC delay times associated with present-day interconnects, to achieve faster signal transmission and thus faster devices. In addition, the use of superconducting interconnects would lead to reduced ohmic (resistive) heating, permitting reductions in distances between device components, which would also lead to faster devices. Significantly, implicit in this proposal, is the assumption that there is no incompatibility between the relatively high T.sub.c superconductors and semiconductors materials, and that no substantial impediments exist to implementation and to the achievement of the underlying advantages.